1. Field of the Invention
The present invention relates to a novel process for preparing styrenic polymer particles and, more particularly, it relates to a process for preparing styrenic polymer particles having a substantially uniform particle size and further to a process for preparing foamable styrenic polymer particles having a substantially uniform particle size.
2. Description of the Prior Art
Styrenic polymers have hitherto been prepared by various polymerization processes such as emulsion polymerization processes and suspension polymerization processes. In the case of directly preparing a granular styrenic polymer, a suspension polymerization process must be employed, but by ordinary suspension polymerization processes it is difficult to prepare styrenic polymer particles having particle sizes in a definite range. That is, in order to suspend a styrenic monomer in the production of polymer particles by a suspension polymerization process, it is necessary to use a suspending agent and to stir the polymerization system. However, in such a case, the sizes of the styrenic polymer particles formed by the suspension polymerization are highly influenced by the kind of suspending agent as well as the stirring conditions, etc. Furthermore, even if the kind of suspending agent and the stirring and other conditions are strictly selected or defined, it is still difficult to produce polymer particles having particle sizes in a definite range by a suspension polymerization process.
In general, if it is possible to make the particle sizes of polymer particles uniform in a definite range, various advantages ensue, e.g., in the case of continuously supplying the styrenic polymer particles to a molding machine (such as an extruding machine), continuous supply of the polymer particles is facilitated and the amount supplied does not vary, which results in moldings having stable qualities. In particular, in producing foamed styrenic resin moldings by processing foamable styrenic polymer particles prepared by impregnating a foaming agent in the polymer particles, there has been an urgent need for polymer particles having uniform particle sizes. This is because if the particle sizes of the foamable polymer particles are too small, the foaming agent contained in the polymer particles readily escapes and thus the foamable polymer particles are apt to lose their foamability during storage, while if the particle sizes of the foamable polymer particles are too large, the packing ratio of the polymer particles is poor in the case of preparing moldings, and thus moldings of a good weld (where the particles melt and adhere well with each other so as to not separate out) are not obtained, and it takes a long time to impregnate the polymer particles with a foaming agent to provide foamable polymer particles. Furthermore, if the particle sizes of foamable polymer particles are uneven, it is difficult to uniformly foam the polymer particles since the foaming time differs between large polymer particles and small polymer particles. For the reasons described above, it has been desired to obtain uniform particles of a mean particle size in the range of 0.8 to 2.5 mm.
However, as mentioned above, it is extremely difficult to obtain polymer particles having a uniform mean particle size by conventional suspension polymerization processes. More practically speaking, the formation of a large proportion of polymer particles having smaller particle sizes than the desired sizes is inevitable with conventional suspension polymerization processes. In particular, it is difficult by conventional suspension polymerization processes to obtain polymer particles having particle sizes larger than 1.5 mm. in good yield, and thus in the case of producing such polymer particles by conventional polymerization processes, polymer particles having the desired particle sizes can be obtained only by sieving off more than a half of the polymer particles obtained by the suspension polymerization and collecting the recovered polymer particles.
In particular, in the case of obtaining foamable polymer particles having desired particle sizes by sieving the foamable polymer particles prepared by a suspension polymerization process, the use of the waste polymer particles for other purposes is greatly limited since they contain a foaming agent.
For the reasons described above, the discovery of a process for producing polymer particles having uniform particle sizes by a suspension polymerization has strongly been desired.